The present invention relates to a demodulator for a frequency shift keyed carrier signal of the type typically employed in data communications and magnetic storage applications. More particularly, the present invention relates to such a demodulator which is digitally implemented and provides increased sensitivity, improved noise immunity, and widened operating temperature range.
Frequency shift keyed (F.S.K.) carried signals are commonly used to transmit digital data from one location to another. That is, an alternating current carrier signal is provided with a number of differing frequencies (two in a binary digital arrangement) respectively representing differing digital states. For example, alternating carrier signals generated to convey binary digital data in one direction over a telephone distribution network, typically have frequencies of 1070 Hz and 1270 Hz representing the two different digital states. Before the digital information represented by such a signal can be utilized at the reception point, it is often necessary to "decode" the frequencies to provide discrete voltage state levels representing the digital data. This conversion typically is achieved by a demodulator especially designed for such purpose, known as a "frequency shift keyed demodulator".
Prior to being fed to such a demodulator for demodulation, F.S.K. carrier signals are conditioned to provide standard logic voltage levels. That is, such a carrier typically is of a low voltage sinusoidal waveform, which is amplified and clipped prior to being demodulated to provide a rectangularly shaped waveform having high and low voltage levels corresponding to the voltage levels representing the digital data.
Most frequency shift keyed demodulators generally employ for such decoding, frequency discrimination characteristics of filters or the like, or phase-locked loop circuitry. However, some F.S.K. demodulators discriminate between the two different frequencies by determining the time intervals between each half-cycle of a carrier signal. Thus, two measurements per full cycle are made. For example, the time between adjacent rising and falling edges of the amplified, rectangular signal discussed above, often is measured. This method offers the possibility of permitting a relatively high transmission rate. The problem, though, is that this method relies on good waveform symmetry, i.e., assumes that the time interval for a half-cycle is equal to one-half the time interval of a full cycle. This is often not the case. Amplifiers are used to condition the signal and generally do not provide a symmetrical waveform gain, with the result that erroneous determinations can be made.